Transportable liquids are important commodities for fuel and chemical use. Currently, liquid hydrocarbons are mostly frequently produced from crude oil-based feedstocks by a variety of processes. However, as the world supplies of crude oil feedstocks decrease, there is a growing need to find alternative sources of liquid energy products. Possible alternate sources include biomass, coal and natural gas. Methane, which is the major constituent of natural gas, biogas and coal gasification is a source along with emulsions including vegetable and animal fats. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil.
Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of natural gas directly to higher hydrocarbons, is a particularly attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome. A large majority of the processes for converting methane to liquid hydrocarbons involve first conversion of the methane to synthesis gas (“syngas” as used herein), a blend of hydrogen and carbon monoxide. Production of synthesis gas is capital and energy intensive; therefore routes that do not require synthesis gas generation are preferred. For example, conventional hydrotreating utilizes two steps, first for production of syngas, and then creation of free radicals under high temperatures and pressure for reaction with oils to be hydrotreated. Such processes are very energy intensive. A number of alternative processes have been proposed for converting methane directly to higher hydrocarbons.
Existing proposals for the conversion of light gases such as methane and carbon dioxide, as well as biofuels, to liquid fuels suffer from a variety of problems that have limited their commercial potential. Oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, frequently require expensive oxygen generation facilities, and produce large quantities of environmentally sensitive carbon oxides. On the other hand, existing reductive coupling techniques frequently have low selectivity to aromatics and may require expensive co-feeds to improve conversion and/or aromatics selectivity. Moreover, any reductive coupling process generates large quantities of hydrogen and, for economic viability, requires a route for effective utilization of the hydrogen byproduct. Since natural gas fields are frequently at remote locations, effective hydrogen utilization can present a substantial challenge.
Another key factor in hydrocarbon liquids is the presence of polynuclear aromatic compounds, as well as total aromatic compounds. In some instances, these compounds are known to be carcinogens. Regulatory agencies have begun to turn their attention to the prevalence of these compounds in the environment and are requiring the reduction of polynuclear aromatics in industrial processes, including fuel processing. Moreover, polynuclear aromatics have a tendency to produce fine particulates when they are combusted, leading to further environmental concerns. However, the reducing of polynuclear aromatic compounds is difficult with existing refining processes because of the variety, technical difficulty and expense of the different reaction pathways required for reduction of polynuclear aromatic compounds. For example, in certain situations, reduction of polynuclear aromatic compounds requires addition of significant quantified of hydrogen gas and results in generation of carbon dioxide, which in itself requires removal and/or remediation.
A particular difficulty in using natural gas as a liquid hydrocarbon source concerns the fact that many natural gas fields around the world contain large quantities, sometimes in excess of 50%, of carbon dioxide. Carbon dioxide is a target of increasing governmental regulation because of its potential contribution to global climate change. In addition, any process that requires separation and disposal of large quantities of carbon dioxide from natural gas is likely to be economically prohibitive. In fact, some natural gas fields have such high carbon dioxide levels as to be currently considered economically unrecoverable.
Similarly, the existing processes for the production of biofuels from fats and oils commonly utilize esterification for the production of Biodiesel, particularly in its unblended form (i.e., B100). This is a costly process, and there are known technical issues with utilizing the Biodiesel, particularly as B100, in existing installations. Embodiments of the invention described below address these issues.
There are also large reserves of heavy oil/bitumen that cannot be readily used. Economically reducing the viscosity (i.e., increasing the API gravity) of heavy oils increases their value to the refiner and also reduces the cost of transportation.
There is also a need to improve the performance of fuels for transportation and heating applications. These improvements include increased efficiency for conversion of the energy to useful work and reduction of emissions of Greenhouse Gases (GHG), including CO2, hydrocarbons, SOX, NOX, and of particulates. Further still, there is need to reduce the aromatic fractions, including polycyclic aromatics, in hydrocarbon fuels and biofuels.
There is a need for an improved process for converting light gas (e.g., methane) to liquid hydrocarbons, particularly where the light gas is present in a natural gas stream containing large quantities of carbon dioxide. There is also a need to create a hybrid fuel to utilize the unique characteristics of products produced from natural gas, bio fats and oils, crude and heavy oil/bitumen in a blended fuel that can be produced at costs comparable with existing hydrocarbon fuels. There is a need for process integration, systems, and apparatus that reduce the total emissions of Greenhouse Gases (GHG) and particulates based on Life Cycle analysis. Such processes also require the potential to utilize carbon dioxide to minimize the emissions thereof.